In order to apply an image to a printing medium inside a printing machine, toner is transferred to the printing medium in a developing device. This printing medium can, for example, be a sheet of paper. If more than one color is to be transferred to the printing medium, several developing devices can be incorporated into the printing machine.
In an electrophotographic printing machine, the toner can be transferred onto a photoelectric drum onto which a latent image previously was exposed. In this process the toner is transferred only onto the exposed areas. Other processes are also possible, in which the toner is transferred only to the unexposed areas. Thereafter, the toner can be further transferred onto a rubber blanket cylinder.
By means of a back-up developing roller and under the influence of an electric field, the toner can be transferred to the printing medium that is being conveyed through the nip between the rubber blanket and the back-up roller.
In order to protect the image that is created in this way from smearing and other damage, the toner is fused onto the printing medium. Fusing of the toner can be accomplished after all of the desired layers of toner have been transferred onto the printing medium. For this purpose, a fusing mechanism, for example, to be located downstream from the last developing device in the printing machine. Alternatively, provision can also be made for fusing the toner downstream from each developing device.
Typical fusing mechanisms fuse the layers of toner on the printing medium by heating the layers of toner and the printing medium. In this process, the toner is heated to a temperature above its glassification temperature and bonded with the printing medium.
Fusing of the toner on the printing medium may often be done inside the fusing mechanism by the application of pressure and heat. For this purpose, a fuser roller and a back-up roller are provided. The printing medium is then fed into the nip formed by the two rollers. The fuser roller and back-up roller are heated for this purpose.
In order to heat the layers of toner and the printing medium, as described in U.S. Pat. No. 6,665,516, issued on Dec. 16, 2003, a microwave mechanism may be used, including a microwave applicator. For this purpose, the printing medium is fed through at least one microwave applicator, in which it and the layers of toner are heated by microwave radiation.
Additional microwave mechanisms may be provided inside a printing machine. For example, microwave mechanisms may be used to preheat the layer of toner before the actual fusing process takes place, which can then be done, for example, by a fuser mechanism containing a fuser roller and a back-up roller.
When microwave mechanisms are used inside the printing machine, the radiation of microwaves can cause undesired effects. For example, the microwave mechanism can be improperly operated or controlled such that the energy level of the microwave radiation is too high. The result can be that parts of the microwave mechanism or a printing medium that is in the microwave mechanism can become overheated. Then it is possible that the printing medium will begin to smolder or even catch fire.
The same undesired effects of the microwave radiation can also occur when the printing medium is subjected to the microwave radiation and remains for a prolonged period of time in the field of the microwave radiation. The energy induced into the printing medium can then be sufficient to cause smoldering or even allow burning to begin.
This prolonged presence in the field can be caused by the printing medium not being aligned properly and lying so crookedly in the printing machine that the printing medium becomes jammed in the printing machine.
It is also conceivable that, based upon a faulty conveyance mechanism or through an incorrect adjustment of the speed of conveyance, the printing medium is conveyed so slowly through the microwave mechanism that in this way, too, the induced energy becomes too great for the printing medium and it begins to smolder or burn.
Electrical discharge can be caused inside the microwave mechanism by dirt accumulations or faulty apparatus. The undesired effect of the microwave radiation can then be that this electrical discharge causes damage to the printing medium (at least point-damage) or peripheral damage to the toner image or to the microwave mechanism itself.